The present invention relates to a platinum supported catalyst for the anode of a PEM fuel cell with high resistance to poisoning by carbon monoxide. The catalyst contains the noble metals platinum and ruthenium on a finely divided, conductive support material. It is particularly suitable for use as an anode catalyst in fuel cells with a polymer electrolyte membrane.
Fuel cells are in principle gas-operated batteries, in which the energy obtained from the reaction of hydrogen and oxygen is converted directly into electrical energy. The present invention describes the preparation of catalysts for fuel cells, in particular the preparation of supported catalysts based on platinum and platinum alloys for PEM fuel cells (PEM=polymer electrode membrane). This type of fuel cell is becoming increasingly important, due to its high energy density and robust structure, for use in the vehicle industry, i.e. for providing electro-traction in motor vehicles.
The advantages of a car running on fuel cells are the very low emissions and the high degree of efficiency as compared with conventional internal combustion engines. If hydrogen is used as the fuel gas, water is produced as the only emission on the cathode side of the cell. The vehicle is then a so-called ZEV (zero emission vehicle). However, hydrogen is still too expensive at the moment and there are problems with the storage and refuelling of vehicles. For this reason the alternative, producing hydrogen directly on board the vehicle by reforming methanol is growing in importance. The methanol stored in the vehicle fuel tank is converted in a steam reforming process at 200 to 300.degree. C. to a hydrogen-rich fuel gas with carbon dioxide and carbon monoxide as secondary constituents. After converting the carbon monoxide by a shift reaction, preferential oxidation (PROX) or another purification process, this fuel gas is supplied directly to the anode side of the PEM fuel cell. Theoretically the reformate gas consists of 75 vol. % hydrogen and 25 vol. % carbon dioxide. In practice, however, this gas also contains nitrogen, oxygen and, depending on the degree of purity, varying amounts of carbon monoxide (up to 1 vol. %).
Supported catalysts based on platinum and platinum alloys are used as catalysts on the anode and cathode sides of PEM fuel cells. These consist of fine, noble metal particles which are deposited onto a conductive support material (generally carbon black or graphite). The concentration of noble metal is between 10 and 40 wt. %, the proportion of conductive support material is thus between 60 and 90 wt. %. The crystallite size of the particles, determined by X-ray diffraction (XRD), is about 2 to 10 nm.
Traditional platinum catalysts are very sensitive to poisoning by carbon monoxide, therefore the CO content of the fuel gas must be lowered to &lt;10 ppm in order to prevent power loss in the fuel cells due to poisoning of the anode catalyst. This applies in particular to a PEM fuel cell which is especially sensitive to CO poisoning as a result of its low operating temperatures of 70 to 100.degree. C.
The present invention is concerned with the preparation of supported catalysts based on platinum and ruthenium which have a high resistance to poisoning by carbon monoxide. CO concentrations of more than 100 ppm in the reformate gas should be possible and should not lead to a noticeable drop in performance of the PEM fuel cell. As a result of using this new type of catalyst on the anode side of the PEM fuel cell the number of process steps to remove carbon monoxide from the fuel gas is reduced. This leads to a considerable decrease in the cost of the system, to an improvement in the efficiency of the system and to a reduction in the size of the overall system. The new catalysts are therefore of great significance with regard to introducing PEM fuel cells into the vehicle industry.
The problem of poisoning of platinum catalysts by carbon monoxide has been recognized for a very long time. CO is adsorbed at the surface of the platinum due to its special molecular structure and thus blocks the access of hydrogen molecules in the fuel gas to the catalytically active platinum centers.
Adsorbed carbon monoxide can be oxidized to carbon dioxide by adding water and can then be removed from the catalyst surface. It is also known that the tolerance of platinum to poisoning by carbon monoxide is improved by alloying or doping the platinum with ruthenium.
L. W. Niedrach et. al. (J. Electrochemical Techn. 5, 1967, page 318) describe the use of Pt/Ru catalysts as CO-tolerant anode catalysts for sulphuric acid fuel cells. These materials consist of fine Pt/Ru alloy powders with high specific surface areas. They are prepared by the so-called ADAMS process from a molten material consisting of platinum chloride, ruthenium and sodium nitrate at 500.degree. C. As a result of the high temperatures during preparation, these catalysts are present as Pt/Ru alloys. The materials are not fixed on a support and are therefore not supported catalysts. Also no information is provided relating to their use in PEM fuel cells.
Pt/Ru supported catalysts have also been available commercially for some time. Thus ETEK Inc., Natick, Mass. (USA) offers corresponding materials for use as anode catalysts in PEM fuel cells.
The catalysts in this case are Pt/Ru alloy catalysts with noble metal concentrations between 5 and 40 wt. % and a Pt/Ru atomic ratio of 1:1. This catalyst has a uniform alloy phase, detectable by XRD. However, it demonstrates unsatisfactory tolerance to carbon monoxide, in particular at concentrations of carbon monoxide of more than 100 ppm and with residual oxygen in the fuel gas.
In a recent paper, M. Iwase and S. Kawatsu report on the production of a CO-tolerant anode catalyst (M. Iwase and S. Kawatsu, Electrochemical Society Proceedings, Vol. 95-23, page 12). In this paper the best results were produced with a Pt/Ru alloy catalyst which was prepared via a special annealing process for producing an alloy. However, the voltage drop with a current density of 0.4 amps/cm.sup.2, at a CO concentration of 100 ppm, is still about 200 mV. This is still too high for practical use. With an unalloyed Pt/Ru system, on the other hand, still poorer results were obtained so it can be assumed from these data that only alloyed Pt/Ru supported catalysts produce the best results for CO-tolerance in a PEM fuel cell.
An object of the present invention is to provide supported catalysts which have an improved tolerance to carbon monoxide, in particular at concentrations of more than 100 ppm. Another object of the present invention is to provide catalysts suitable for use with carbon monoxide, nitrogen and oxygen-containing fuel gases that exhibit the lowest possible voltage drop with high current densities.